Apparatus for transferring heat



Nov. 28, 1939. M. E. F lENE APPARATUS FOR TRANSFERRING HEAT Filed April24, 1937 0 45 so :20 m: :r: m 13! METAL SURFACE TEMPERATURE DEG. C.

Inventor: Marcus E. Fiene,

1 W 5 jwlflq His Attor'n Patented Nov. 28, 1939 UNITED STATES PATENTOFFICE Marcus E. Fiene, Ballston Lake, N. Y., assignor to GeneralElectric Company, a corporation of New York Application April 24, 1937,Serial No. 138,789

2 Claims.

This invention relates to methods and apparatus for transferring heatfrom a relatively high temperature source to a vaporizable medium underrelatively low pressure particularly subatmospheric pressure where theusual and well known laws of heat transmission fail to apply.

The invention provides improvements whereby I heat is transmittedreadily to vaporizable mediums at relatively low pressures such as waterthrough heat conducting bodies such as iron, steel or other metals evenwhen the temperature of the heat source is above the critical value atwhich the water will assume a spheroidal state in which it ordinarily isexceedingly diflicult to transmit heat thereto to vaporize it. With thevaporizable medium in a spheroidal state, the usual and well I knownlaws of heat transmission no longer apply.

Such phenomenon occurs whenever a drop of water is placed onthe top of avery hot stove and may be termed non-wetting, i. e., the water isprevented from contacting and wetting the surface of the heated body andhence cannot receive any appreciable amount of heat therefrom.

The improvements of the present invention are of particular advantage inevacuated vapor heat transfer systems of the type disclosed in mycopending application, Serial No. 25,691, filed June 8, 1935. Thisevacuated system consists briefly of a vaporizer, condenser-radiator,and thermostatically controlled means for varying the amount ofvaporizable liquid active in transferring heat. With the relatively lowsub-atmospheric pressure and the relatively small amounts of vaporizableliquid involved in such an evacuated system, the above describednontting ordinarily occurs whenever the vaporizer is raised above thecritical temperature. Thus the heat transfer system may functionentirely satisfactorily with low pressure and consequently lowtemperature steam for heating the vaporizer but when high pressuresteam, which necessarily is at a higher temperature, is used as thesource of heat no substantial heat transfer may be obtained.

It is the primary object of the present invention 45 to provide a methodof and apparatus for transferring heat toa vaporizable medium in whichthe difliculties arising from non-wetting under the unusual heattransfer conditions mentioned above are effectively overcome.

Briefly, the invention consists in conducting heat from the hightemperature-heat source to the vaporizable medium through a first heatconducting body having a substantially large heat conducting and storagecapacity and then through a 55 body having a relatively slight heatconducting and storage capacity. In actual practice these heatconducting. bodies may be formed of an integral body with a surface onone side adapted to be contacted by the high temperature heat source anda ridged surface on the other side with 5 the ridges of substantiallytriangular cross-section adapted to be contacted by the vaporizablemedium. When the present invention is applied to a vapor heat transfersystem of the type noted 10 above, steam will contact with the heatconducting body or vaporizer. With no vaporizable medium in thevaporizer, there will be no appreciable temperature drop in thevaporizer and the walls thereof will be maintained at substantially the15 temperature of the steam. When a vaporizable medium, such as water,is admitted into the vaporizer part of it will contact the groove formedbetween the ridges and thereby contact the body having a relativelylarge heat conducting and 20 storage capacity. The temperature of thisbody remains substantially unchanged and, when above the critical valueat which the water assumes a spheroidal state, non-wetting occurs.However, some of the water will contact the outwardly 25 extendingportions such as the apices of the triangular ridges which have only aslight heat conducting and storage capacity. Due to the transfer of heatfrom the apices of the ridges to the water, the temperature of theapices of the ridges so rapidly drops to a value low enough to-preventnon-wetting, i. e., allows contact between the water and heat conductingbody, with the result that heat is transmitted readily to the water andthe latter is vaporized.

The above referred to ridges may assume various shapes, as will bedisclosed hereinafter, and all forms equivalent to those described areintended to be within the scope of this invention.

A further object of myinvention is so to form 40 the ridges of theportion of the heat conducting bodies having large and small heatstorage capac-- lties, respectively, that heat transfer occurs at asubstantially constant rate irrespective of the temperature, of thesource of heat. In other words, lain-object of this invention is to soproportion the areas that they provide the equivalent of a'steampressure reducing valve in the conventional'steam heating system.

The portion of the ridged memberin a nonwetting condition will dependupon its shape, the pressure of the vaporizable medium and thetemperatureof the source. At relatively low pressures and hightemperatures only a small portion will transfer an appreciable amount ofheat to the water and at relatively high pressures and low temperaturesthe area in contact with the water will increase. The rate of heattransfer may, therefore, be designed to remain substantially independentof the temperature. The results described immediately above may beobtained by forming the heat conducting body with ridges over only aportion of its surface. According to this arrangement the ridged portionwould be wetted at low pressures and high temperatures and the entirebody at high pressures and low temperatures.

Further objects and advantages of my invention will become apparent asthe following description proceeds, and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to theaccompanying drawing in which Fig. 1 shows, partly in section, an airheating radiator unit deriving its supply of heat through an evacuatedvapor heat transfer system embodying my invention; Fig. 2 is a partialcrosssection view of the novel heat exchanger of my invention utilizedin the system disclosed in Fig. 1;

Fig. 3 discloses another form of the novel heat exchanger of myinvention applied to a water heating system; and Fig. 4 shows a seriesof experimentally determined curves defining pressuretemperature regionsin which boiling and nonwetting occur in an experimental set-up designedto approximate actual apparatus.

In order to better illustrate the phenomenon of non-wetting, referencewill be made first to Fig. 4 which shows a series of curves obtainedwhen water was dripped onto a heated brass plate under a bell jar inwhich the pressure could i be varied to a relatively low pressure, anexperimental set-up approximating conditions obtaining in a vaporgenerating system of the usual type. The brass plate was held at anangle of approximately 20? during the course of the experiment. Theabscissae represent the surface temperature of the plate and theordinates represent the absolute pressure under the bell jar in inchesof mercury. The curve A indicates the normal vapor pressure curve andthe area to the left thereof indicates a region of no boiling, i. e.,when the plate was at the temperature indicated by the abscissa and thevacuum as indicated, no boiling took place. The area between curves Aand B indicates the area in which boiling was obtained and the surfaceof the plate was wetted. The area between curves B and C indicates aregion in which explosive boiling occurred and the surface was onlypartially wetted. The area to the right of curve C indicates a region ofno boiling and in which non-wetting occurred. Thus the curve 0represents the critical temperaturepressure range beyond whichnon-wetting always occurs with very little, if any, conduction of heatbetween the plate and the water, the effective heat transfer occurringby radiation through the space between the plate and the water.

While I am about to describe my invention in connection with a vaporheat transfer system of j transferredfrom a relatively high temperaturesource to a vaporizable medium under relatively low pressure. In theillustrated embodiment there is provided a radiator-condenser i l f rheating air within an enclosure indicated at II. This radiator-condenseris preferably formed of suitable pressed metal plates welded together toform a series of interconnected vapor condensing columns I2 with openair circulation passages l3 therebetween' to facilitate dissipation ofheat from the radiator to the air. The bottom wall of the radiator ispreferably sloped so as to readily drain the condensed vapor into theconduitshaped receiving tube It. Tube I4 is joined with a vaporizingtube l5 having one end thereof sloping downward and extending into thesteam chest IS in which live steam is maintained at all times. Thedownwardly sloping portion I! of the vaporizing tube l 5 within thesteam chest is made with a substantially smooth exterior l8 adapted tobe contacted by the steam and the interior of the tube is provided withone portion substantially smooth, indicated at [9 and a second portionprovided with a series of grooves as indicated at 20. The constructionof the grooved portion is illustrated in greater detail in Fig. 2.

Steam is supplied from any suitable boilenor source (not shown) by thesteam supply pipe 2| which also serves 'to return the condensed steam tothe source. A suitable heat insulating cover 22 effectively prevents anydissipation of heat from the steam chest IS, the steam pipe 2|, as wellas from the vaporizing tube l5 except through the operation of the heattransfer system in a manner to be described hereinafter.

A closed liquid control chamber 23 is located remotely from both thevaporizing surfaces of the tube I 5 and the condenser-radiator l0 andhas a relatively small size tube 24 communicating with the bottomthereof and the bottom of the condensate receiving tube I 4. This servesto minimize the transfer of heat between the control chamber 23 and themain heat transfer system consisting of the vaporizer tube and thecondenser-radiator.

An electrical heating unit 24 of the cartridge type is mounted insidethe tube 25 which is sealed into the liquid control chamber 23. Theelectrical heating unit is of relatively small capacity and is energizedat low voltage derived from the secondary of the transformer 26 to whichit is connected selectively by thermal responsive means 21 positionedsuitably within the enclo-' cooperatively associated therewith andcarried by a bimetallic thermal responsive element 29. I

Associated with the thermostat is a preheat coil locally influencing thelatter,- such as a resistor 30, which is connected at one end of thebimetallic element and at the other by a conductor 3| to the secondarywinding. These 'connectionsare such that the preheat, coil is ener dw vethe contacts are closed by the room thermostat to energize the cartridgeheater.

The capacity of the cartridge heater is suchas to expel all the liquidfrom the control chamber in fifteen or twenty minutesl .Thus afterenergization thereof a portion of ,the'fluid isexpelled and the longerthe heater is energized the: more fluidwill be expelled with the resultv. that a H greater area of the vaporizing surface is wetted and thetemperatures of the control chamber and condenser, surface rise. Afterthe temperature of the space reaches a desired value the thermo stat,deenerg'izes the cartridge uheaterallowii'ngf the temperature of thecontrol chamber andtliecondenser surface to fall; The. control chamberis so constructed that its rate of heat dissipation under normalconditions will reduce its temperature to approximately room value infifteen or twenty minutes. Modulated control is obtained by providingthe control thermostat with the preheatcoil 30 for locally heating it.Application of preheat to the thermostat whenever the thermostat callsfor heat causes the latter to operate at relatively frequent intervals,dependent upon the amount of preheat, intermittently energizing thecartridge heater. Due to this intermittent heating and the heat storagecapacity of the control chamber the temperature of the liquid ismaintained at an average varying but slightly from that predeterminedvalue at which heat is transmitted to the heated space continuously andat a rate just sumcient to overcome losses due to dissipation.

Referring now to Fig. 2, a more complete description of the novel heatconducting body of my invention will be given. In this figure isillustrated in enlarged cross-section the ridged portion 20 of thevaporizing tube 15. It will be seen from this figure that the outersurface l8 thereof is relatively smooth, while the inner surface isridged. The individual ridges 33 are of substantially triangularcross-section and terminate in a sharp apex and define between them aplurality of relatively flat sections 34. The grooves in Fig. 2 areillustrated as being threaded in a continuous well known manner, butthey may just as well be formed circumferentially. The ridges are alsoillustrated as triangular in crosssection but they need not be made inthis fashion. The criterion is that the ridges should provide bodiesadapted to be contacted by the vaporizable medium and when contactedtheir temperature should be subject to considerable change so that thetemperatures of the extremities will decrease below that critical valueat which nonwetting occurs. Thus, these grooves may be madesubstantially triangular in cross-section or even rectangular provided,however, that the projecting portions or ridges 33 shall have a slightheat conducting and storage capacity as compared with the main body ofthe heat conductor I9.

The figure shows a. preferred method of constructing the grooves but itis obvious that they could be made by forming the ridged portionsseparately and welding or otherwise securing them to a cylindrical bodyor uniting a number of bodies through which the heat flows in such adirection that the heat flow will be first through a body having arelatively great heat storage capacity and then through a body having arelatively slight heat storage capacity.

In operation, whenever the temperature within the enclosure ll decreasesbelow a predetermined value, thermostatic element 29 closes a circuitthrough the heater 24 across the secondary winding of transformer 26 andsimultaneously energizes the preheat coil 30. Energization of heater 24creates a vapor pressure within the control chamber 23 thereby forcing aportion of the vaporizable liquid, such as water, into the vaporizingtube l5. With steam supplied to the steam chest l6 at all times, thevaporizable medium is transformed into vapor and the vapor flows throughtubes l5 and I4 to the condenserradiator l where it is condensed and theheat absorbed by the radiating surface and transmitted to the air withinthe enclosure. The temperature within the enclosure rises and the amountof vaporizable medium in the heat transfer system will be determined bythe temperature requirements within the enclosure. The greater thedemand for heat, the greater the length of time that the cartridgeheater isenergized and consequently the more vaporizable medium in thesystem. The more liquid in the system, the higher the temperature andpressure to which the medium is raised and consequently the higher thetemperature at the condenser-radiator. Thus, the amount of heatavailable is, dependent upon the demand and a modulated. supply of heatis obtained. Whenever the heating requirements decrease the amount ofheat supplied to the cartridge heater is decreased and the temperaturewithin the control chamber and condenser-radiator decreases. The preheatcoil acts to shorten the energization period of the cartridge heater togive a closer modulation of the temperature.

In some installations the steam supplied to the steam chest l6 throughthe supply pipe 2| may undergo considerable variations in temperature.At times it will be below that critical value at which the vapor withinthe vaporizer is transformed intoa spheroidal condition and at times itwill transform the vapor into a spheroidal state and thereby causenon-wetting. However, by the use of a vaporizer constructed inaccordance with my invention, the non-wetting of the entire surface isprevented. The mode of operation inthis case is as follows: .Wheneverthe vaporizable medium is forced into the vaporizer a portion thereofcontacts the apices 33 of the ridges and another portion reaches theflat parts 34. The latter is transformed into a spheroidal conditionwith substantially no heatbeing transmitted to them. The liquidcontacting the apices will, however, decrease the temperature thereof toa point where the surface is wetted and heat readily transmitted to theliquidthereon to vaporize it. The extent of wetting on the ridges isdependent upon the temperature of the steam within the steam chest. Atvery high temperatures the apices-alone will be wetted and withdecreasing temperatures an increased area of the ridges will be wettedand at temperatures below the critical value the entire inner surface ofthe vaporizer will be wetted and heat transmitted to the liquidthroughout the whole chamber or through a zone that shifts from theapices I to the main body of the vaporizer. The arrangement thereforeprovides a heating system which automatically acts as a reducing valvebetween the source of supply of heat and the vaporizer for the reasonthat the water may be boiled irrespective of the temperature of theprimary source of steam. y

In Fig. 3 I have illustrated a modified form of a vaporizer and aslightly different form of vapor heat transfer system that isparticularly adapted for heating a supply of water. Thecondenserradiator of the system is diagrammatically illustrated as acoil 40 positioned within a heat exchanger tank Ill and connected bymeans of a vapor inlet tube 42 to the vaporizing chamber 43. Thecondensate from the condenser-radiator is returned by conduit 44 to acondensate storage tank 45 connected by a conduit 46 to the bottom ofthe vaporizing chamber 43. The conduit 46 has interposed therein a valve41 normally biased to its open position by a spring 48 and adapted to beclosed by the energization of a solenoid 49 under the control of athermal responsive means 50 whenever the latter closes a circuit acrossthe secondary winding of a transformer The thermal responsive means 50is responsive to the temperature of water within the water storage tank52 which is connected by conduits 53 and 53' to the heat exchanger tank4|.

Within the vaporizing chamber 43 is the novel vaporizer 54 of myinvention consisting, in this modification, of a tube having arelatively smooth liquid contacting portion 55 and a ridged liquidcontacting portion 56 so proportioned that each is about one-half thearea of the vaporizer. Steam is supplied to the vaporizer through aconduit 51 from any suitable source and the condensed steam is led fromthe steam chest through a return conduit 58. The heat losses from thevaporizing chamber are kept as low as possible by heat insulatingmaterial 59 surrounding the chamber.

The ridged portion of the vaporizer 54 is constructed substantially asillustrated in the modiflcation shown in detail in Fig. 2 with theexception that the ridges in this case are on the outer side of thetube. It should be noted, however, that the ridges are adapted tocontact the vaporizable medium as in the previously describedmodification. The operation of the vaporizer of this modification is, inall respects, the same as that of the previously described modification.When extremely high temperature steam is supplied to the vaporizer onlythe ridged portion 56 is wetted and when comparatively low temperaturesteam is supplied to the vaporizer the entire surface is wetted. Whenthe temperature of the water inthe storage tank 52 rises to apredetermined value the thermal responsive means 50 energizes solenoid49 to close valve 48. The latter cuts off the supply of condensed liquidto the vaporizer and consequently the transfer of heat to the water intank 4| by the condenser-radiator 40. When the temperature within thetankfalls below the desired value the thermal responsive meansdeenergizes the solenoid 49 so that the valve may be opened by spring 48and liquid again supplied to the vaporizer.

In actual practice the portions 33 and 56 have been made in ridges ofsubstantially triangular cross-section having a height of .105", a baseof a body of vaporizable liquid under relatively low pressure to saidvaporizing chamber and in contiguity to said wall, means for supplying avaporized fluid at relatively high temperature and pressure to saidcondensing chamber to maintain said wall at a temperature above thecritical nonwetting temperature-pressure range within said vaporizingchamber, and a plurality of projections extending from said wall andhaving the ends thereof of relatively slight heat conducting and storagecapacity for engaging with said body of vaporizable liquid to reduce thetemperature of said end portion below the critical pressuretemperaturerange and thereby provide a wetting heat conducting contact with saidbody of vaporizable liquid.

2. In combination, a steam chamber having means for supplying steam atvariable temperature and pressure thereto, an evacuated heat transferenclosure having a vaporizer in heat transfer relation with said steamchamber and provided with means 'for supplying a body of vaporizableliquid to said vaporizer under vacuum, said vaporizer having a pluralityof projections of slight heat conducting and storage capacity extendingfrom a portion of the surface thereof into engagement with said body ofvaporizable liquid for establishing a wetting heat transfer contacttherewith when the steam supplied to said steam chamber raises thetemperature of said vaporizer above the critical temperature-pressurerange.

MARCUS E. .FIENE. 45

